1. Field of the Invention
The present invention relates to novel ferroelectric side-chain liquid crystalline copolymers, with siloxane backbone and a triaromatic mesogen as the side group. In these copolymers, a certain proportion of the repetitive units of the polymer backbone bears the mesogenic side groups. The materials exhibit ferroelectric smectic C (or SmC*) mesophase over a large range of temperatures, in some cases extending to subambient temperature. They also possess high values of spontaneous polarization and exhibit fast electro-optic switching times. In addition, these materials show pronounced electroclinic effects in the smectic A phase. Such liquid crystalline compounds are useful in the field of electro-optic devices, pyro-electric and piezo-electric detectors, non-linear optics, etc.
2. Discussion of the Background
In the last decade, the area of applications of liquid crystal displays (LCDs) has grown from wrist-watches to computer terminals and television displays. Most of the current liquid crystal displays are based on effects shown by conventional low molecular weight nematic liquid crystals. However, these materials are limited in that their response speeds are low, i.e., on the order of several tens of milliseconds. To overcome this problem, Clark and Lagerwall described a structure in which the ferroelectric character of the chiral smectic phase (SmC*) is optimally employed: the so-called Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) structure. These liquid crystals have bistable states under the application of an electric field, the states being switchable on reversing the polarity of the field. Since this switching is essentially related to molecular rotation about its long axis, the response speed is faster than in the case of a nematic liquid crystal.
Still, the application of these materials for the processing of large size and curved screens remains limited. This is because, in order to make these large screens, it is important to have a uniform and defined distance (.about.microns) between the two glass substrates in which the liquid crystalline compound is sandwiched. Practically, this is impossible to accomplish over large areas.
In order to solve this problem, it has been tried to use polymeric liquid crystals so as to make the materials easily processable (molding, film making, etc.). There have been some reports of the observation of the SmC* phase in side-chain liquid crystal polymers.
For example, Shibaev et al, Polymer Bull., vol. 12, p. 299 (1984) have reported a side chain liquid crystal polymer, and the exact formula: ##STR2## was reported later by Shivaev et al, Pure and Applied Chem., vol. 57, p. 1589 (1985).
Decobert et al, Polym. Bull., vol. 14, p. 179 (1985) have reported a number of polymers having the structure AX.sub.n : ##STR3## X.dbd.H, CH.sub.3, Cl n=2, 6, 11.
Guglielminetti et al, Polym. Bull., vol. 16, p. 411 (1986) have reported compounds of the formula BX.sub.n ##STR4## X.dbd.H, CH.sub.3, Cl n=2, 6, 11
Shibaev et al, Vissokomol Soedin, vol. 29, p. 1470 (1987) have also reported a side chain liquid crystal polymer of the formula: ##STR5##
Hahn and Percec, Macromolecules, vol. 20, p. 2961 (1987) have reported a polysiloxane side chain liquid crystal polymer having the formula: ##STR6##
Keller, Ferroelectrics, vol. 85, p. 813, (1988) has described another polysiloxane liquid crystal polymer of the formula ##STR7##
Zentel et al, Liquid Crystals, vol. 2, p. 83, (1987) have prepared combined liquid crystal polymers of the formula ##STR8## n=2 or 6
Bualek et al, Makromol. Chem., vol. 189, p. 797 (1988) have also prepared combined liquid crystal polymers which have the formula: ##STR9##
Kapitza et al, Makromol. Chem., vol. 189, p. 1793 (1988) have reported combined liquid crystal polymers of the formula: ##STR10##
However, these materials exhibit the SmC* phase only at high temperatures. Also, the materials reported so far have switching times of the order of a few milliseconds and hence are not suitable for the above-described applications. Thus, there remains is a need for faster switching polymers with ferroelectric phase over a large temperature range extending to room temperature. The present invention reports novel materials which satisfy these requirements.